add $nu.data-dir for completions and $nu.cache-dir for other uses (#13122)

# Description

This PR is an attempt to add a standard location for people to put
completions in. I saw this topic come up again recently and IIRC we
decided to create a standard location. I used the dirs-next crate to
dictate where these locations are. I know some people won't like that
but at least this gets the ball rolling in a direction that has a
standard directory.

This is what the default NU_LIB_DIRS looks like now in the
default_env.nu. It should also be like this when starting nushell with
`nu -n`
```nushell
$env.NU_LIB_DIRS = [
    ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
    ($nu.data-dir | path join 'completions') # default home for nushell completions
]
```

I also added these default folders to the `$nu` variable so now there is
`$nu.data-path` and `$nu.cache-path`.

## Data Dir Default

![image](https://github.com/nushell/nushell/assets/343840/aeeb7cd6-17b4-43e8-bb6f-986a0c7fce23)

While I was in there, I also decided to add a cache dir

## Cache Dir Default

![image](https://github.com/nushell/nushell/assets/343840/87dead66-4911-4f67-bfb2-acb16f386674)

### This is what the default looks like in Ubuntu.

![image](https://github.com/nushell/nushell/assets/343840/bca8eae8-8c18-47e8-b64f-3efe34f0004f)

### This is what it looks like with XDG_CACHE_HOME and XDG_DATA_HOME
overridden
```nushell
XDG_DATA_HOME=/tmp/data_home XDG_CACHE_HOME=/tmp/cache_home cargo r
```

![image](https://github.com/nushell/nushell/assets/343840/fae86d50-9821-41f1-868e-3814eca3730b)

### This is what the defaults look like in Windows (username scrubbed to
protect the innocent)

![image](https://github.com/nushell/nushell/assets/343840/3ebdb5cd-0150-448c-aff5-c57053e4788a)

How my NU_LIB_DIRS is set in the images above
```nushell
$env.NU_LIB_DIRS = [
    ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
    '/Users/fdncred/src/nu_scripts'
    ($nu.config-path | path dirname)
    ($nu.data-dir | path join 'completions') # default home for nushell completions
]
```

Let the debate begin.

# User-Facing Changes
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# Tests + Formatting
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crates/nu-protocol/src/eval_const.rs

@@ -149,6 +149,38 @@ pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Valu
         },
     );
 
+    record.push(
+        "data-dir",
+        if let Some(path) = nu_path::data_dir() {
+            let mut canon_data_path = canonicalize_path(engine_state, &path);
+            canon_data_path.push("nushell");
+            Value::string(canon_data_path.to_string_lossy(), span)
+        } else {
+            Value::error(
+                ShellError::IOError {
+                    msg: "Could not get data path".into(),
+                },
+                span,
+            )
+        },
+    );
+
+    record.push(
+        "cache-dir",
+        if let Some(path) = nu_path::cache_dir() {
+            let mut canon_cache_path = canonicalize_path(engine_state, &path);
+            canon_cache_path.push("nushell");
+            Value::string(canon_cache_path.to_string_lossy(), span)
+        } else {
+            Value::error(
+                ShellError::IOError {
+                    msg: "Could not get cache path".into(),
+                },
+                span,
+            )
+        },
+    );
+
     record.push("temp-path", {
         let canon_temp_path = canonicalize_path(engine_state, &std::env::temp_dir());
         Value::string(canon_temp_path.to_string_lossy(), span)